In many polyolefin polymerization processes, catalyst is prepared for delivery to the polymerization process by activating the catalyst solids in a continuous process commonly referred to as pre-contacting. This pre-contacting step was developed to ensure that the catalyst solids, such as a Ziegler-Natta “Generation III, IV or V” catalyst with a titanium polymerization site in its Ti(IV) state, were activated via an activator, such as a metal alkyl, e.g., triethylaluminum (“TEA1”), to reduce the titanium polymerization site to its active Ti(III) state.
Pre-contacting is often accomplished by introduction of the catalyst (in the form of catalyst solids) and the activator (sometimes diluted in an alkane solvent, such as hexane) into a pre-contacting vessel, where the catalyst and activator are allowed to mix and react for a pre-contacting time of typically 10 to 20 minutes outside the presence of the olefin monomers. In some processes, the catalyst is further pre-contacted with an external donor system to ensure thorough complexation of the external donor with the active polymerization site to influence the stereoregularity of the resulting polyolefin. In such cases, the external donor may be diluted in a solvent, such as mineral oil, prior to being added to the pre-contacting vessel.
If the catalyst system is not sufficiently active, low levels of pre-polymerization are observed during the subsequent pre-polymerization step. It has been demonstrated that low levels of pre-polymerization under the mild pre-polymerization conditions can translate to poor product morphology, such as broken granules, low bulk density and high fines content. In addition, insufficient time for external donor complex formation can result in reduced product crystallinity.
Injection of the activated catalyst system into the pre-polymerization reactor has been and continues to become more challenging as the newer Ziegler-Natta catalyst systems achieve higher levels of activity. Even under the very mild conditions where the active catalyst first comes into contact with monomer, the activity can be high enough to cause polymer formation and plugging in the injection systems, resulting in reduced reliability of the production facility and other issues associated with injector plugging and reactor fouling.
Most of the reliability improvement development work in this area has been directed to tempering the reaction conditions at the point where the active catalyst solids first contact monomer, including colder injection conditions (e.g., reducing the monomer feedstream temperature to as low as −20° C. or lower); higher monomer feedstream flow velocities; or monomer dilution (e.g., with propane or other inert hydrocarbons). Each of these solutions has associated disadvantages, including increased operating costs, lower process efficiency, and reduced catalyst productivity. It would therefore be desirable to provide a solution to the problems associated with the use of high-activity catalysts in such polyolefin polymerization processes while reducing or eliminating such disadvantages.